Since she's undercover, it's important that her magic items not be obvious as magic items. That eliminates things such as weapons & armor (she may be able to carry those openly in a viking culture, but they're little help for subterfuge), scrolls, potions, rods, staves, and wands. Only wondrous items are left, and many of those aren't subtle enough either.

Consider giving her eyes of doom to use on characters who politically oppose her puppets. Nothing would embarrass a viking chief more than a sudden bout of fear, and being shaken affects almost all of his dice rolls to boot.

If she's spending most of her time in her thrall's tent, she could have one of the magic items that lets her summon an elemental (a bowl, brazier, censer, or stone) stashed in there. Remember, the range of a summon spell and the caster level of the item would make it appear a ways off from where she is. Having an elemental suddenly appear and go on a rampage can be a good distraction if necessary (especially if the vikings are superstitious), and is a suspicion-free way to bump off a foe (until the PCs locate the magic item as evidence against her).

It's a bit expensive, but a mirror of opposition lets her create "evil twins" of those who see it after she speaks the activating command word. She'd have to make sure the original is kept alive (since its death destroys the duplicate), but what better way to replace annoying meddlers than with duplicates, instead of just spamming them with charm monster all the time. And if she's discovered, this can cover her getaway/counterattack.

An orb of storms is a nice magic item for plausible deniability. Controlling the weather is a great "hands off" way to affect things...and when lightning suddenly strikes someone dead out of nowhere, surely the gods themselves must have done it.

Hope those help!